Restoring prolonged standing via functional electrical stimulation after spinal cord injury: A systematic review of control strategies

Perspectives of current and future use of electrical stimulation home-devices from people with spinal cord injuries and healthcare professionals

INTRODUCTION

Electrical stimulation (E-stim) can reduce the impact of complications, like spasticity, bladder dysfunction in people with spinal cord injuries (SCIs), enhancing quality of life and health outcomes. With SCI prevalence high in regional Australia and a shift towards home-based community integrated care, the perspectives of people with SCI and healthcare professionals on current and future use of E-stim home-devices are needed.

METHODS

A mixed-methods concurrent triangulation approach was used. A cross-sectional survey collected demographics and perspectives on E-stim technology from 84 individuals with SCI and healthcare professionals. Focus groups with 36 participants explored the experiences, facilitators, and barriers of using E-stim home-devices.

RESULTS

Healthcare professionals prioritised the use of E-stim for muscle strength and functional improvements, whereas people with SCI prioritised alleviating secondary complications. Healthcare professionals emphasised personal responsibility; however, people with SCI saw the clinical relationship as an important facilitator. Both groups agreed on the cost, accessibility, and funding as major barriers.

CONCLUSIONS

Different expectations between healthcare professionals and people with SCI highlight the need for improved communication when establishing goals and expected outcomes. Successful E-stim home-device implementation relies on device availability and tailoring education to suit people with SCI and healthcare professionals.

Effectiveness of FES-supported leg exercise for promotion of paralysed lower limb muscle and bone health-a systematic review

Leg exercises through standing, cycling and walking with/without FES may be used to preserve lower limb muscle and bone health in persons with physical disability due to SCI. This study sought to examine the effectiveness of leg exercises on bone mineral density and muscle cross-sectional area based on their clinical efficacy in persons with SCI. Several literature databases were searched for potential eligible studies from the earliest return date to January 2022. The primary outcome targeted was the change in muscle mass/volume and bone mineral density as measured by CT, MRI and similar devices. Relevant studies indicated that persons with SCI that undertook FES- and frame-supported leg exercise exhibited better improvement in muscle and bone health preservation in comparison to those who were confined to frame-assisted leg exercise only. However, this observation is only valid for exercise initiated early (i.e., within 3 months after injury) and for ≥30 min/day for ≥ thrice a week and for up to 24 months or as long as desired and/or tolerable. Consequently, apart from the positive psychological effects on the users, leg exercise may reduce fracture rate and its effectiveness may be improved if augmented with FES.

Control strategy for intraspinal microstimulation based on central pattern generator

Intraspinal microstimulation (ISMS) is considered as a special functional electrical stimulation (FES) method. This method can restore the movement of paralyzed limbs in patients with spinal cord injury (SCI) using electrical stimulation of spinal cord. There is a special site for central pattern generator (CPG) in the spinal cord. The ISMS acts on the CPG site, and single electrode stimulation produces alternating motion in the hindlimbs of SCI rats. Based on the long short-term memory network (LSTM), a mapping model was established between the stimulation intensity of specific CPG sites and the angle of the knee joint to reflect the motor characteristics of the rat hindlimb. We proposed an LSTM-iterative learning control (ILC) strategy to form a closed-loop control to accurately control hindlimb movement. The proposed LSTM model fits the actual joint angle curve well, and the LSTM-ILC strategy can accurately regulate the hindlimb movement, allowing rats to perform rehabilitation training based on pre-set knee trajectories.

Transcutaneous Functional Electrical Stimulation Controlled by a System of Sensors for the Lower Limbs: A Systematic Review

In the field of transcutaneous functional electrical stimulation (FES), open-loop and closed-loop control strategies have been developed to restore functions of the lower limbs: walking, standing up, maintaining posture, and cycling. These strategies require sensors that provide feedback information on muscle activity or biomechanics of movement. Since muscle response induced by transcutaneous FES is nonlinear, time-varying, and dependent on muscle fatigue evolution, the choice of sensor type and control strategy becomes critical. The main objective of this review is to provide state-of-the-art, emerging, current, and previous solutions in terms of control strategies. Focus is given on transcutaneous FES systems for the lower limbs. Using Compendex and Inspec databases, a total of 135 review and conference articles were included in this review. Recent studies mainly use inertial sensors, although the use of electromyograms for lower limbs has become more frequent. Currently, several researchers are opting for nonlinear controllers to overcome the nonlinear and time-varying effects of FES. More development is needed in the field of systems using inertial sensors for nonlinear control. Further studies are needed to validate nonlinear control systems in patients with neuromuscular disorders.

Gait regulation of hindlimb based on central pattern generator in rats with a spinal cord injury

A spinal stimulator that can regulate hindlimb movements using monopolar stimulation has not been developed yet. Nevertheless, in a previous study, we found a specific central pattern generator site on the right side of the rat spinal cord. By stimulating these sites with certain pulse signals, the alternating movement of the hindlimb can be obtained using fewer electrodes. Therefore, in this research, considering the specific central pattern generator site as the target, functional electrical stimulation was performed on rats with spinal cord injury using monopolar stimulation. Angle sensors were used to track and capture the knee joint angle data of the right hindlimb; thus, the mapping relationship between the voltage amplitude and the knee angle parameters was established. Based on this relationship, the rats’ hindlimb were controlled. Compared with the traditional spinal stimulator, the proposed approach increases the gait feedback, requires fewer electrodes, and simplifies the timing of stimulation. The rats with spinal cord injury were subjected to stimulation training for half an hour every day for 28 consecutive days. The Basso, Beattie and Bresnahan score showed that 76% of the health level could be achieved on the 28th day. Finally, somatosensory evoked potential analysis showed that the measurement results were close to the standard value on the 28th day. This study lays a foundation for future rehabilitation research on the hindlimb.

An Iterative Learning Controller for a Switched Cooperative Allocation Strategy during Sit-to-Stand Tasks with a Hybrid Exoskeleton

A hybrid exoskeleton that combines functional electrical stimulation (FES) and a powered exoskeleton is an emerging technology for assisting people with mobility disorders. The cooperative use of FES and the exoskeleton allows active muscle contractions via FES while robustifying torque generation to reduce FES-induced muscle fatigue. In this paper, a switched distribution of allocation ratios between FES and electric motors in a closed-loop adaptive control design is explored for the first time. The new controller uses an iterative learning neural network (NN)-based control law to compensate for structured and unstructured parametric uncertainties in the hybrid exoskeleton model. A discrete Lyapunov-like stability analysis that uses a common energy function proves asymptotic stability for the switched system with iterative learning update laws. Five human participants, including a person with complete spinal cord injury, performed sit-to-stand tasks with the new controller. The experimental results showed that the synthesized controller, in a few iterations, reduced the root mean square error between desired positions and actual positions of the knee and hip joints by 46.20% and 53.34%, respectively. The sit-to-stand experimental results also show that the proposed NN-based iterative learning control (NNILC) approach can recover the asymptotically trajectory tracking performance despite the switching of allocation levels between FES and electric motor. Compared to a proportional-derivative controller and traditional iterative learning control, the findings showed that the new controller can potentially simplify the clinical implementation of the hybrid exoskeleton with minimal parameters tuning.

Impedance Control of Human Ankle Joint With Electrically Stimulated Antagonistic Muscle Co-Contraction

Functional electrical stimulation (FES) is often used, typically in an open-loop manner, to restore paralyzed motor function for daily living activities. Several feedback control strategies have been developed to improve the performance and usability of FES-evoked movement. However, most of them have been position controllers, while the control strategy for human movement has been known as impedance modulation. Moreover, few studies have attempted to use antagonistic co-contraction for FES feedback control despite its expected benefits, such as enhanced stability and performance and better rehabilitation outcome. In this paper, we propose a robust impedance controller for FES that can adjust the intrinsic joint stiffness using co-contraction. It consists of an impedance control law based on time-delay estimation to compensate for the nonlinear uncertain joint dynamics and an antagonistic muscle co-contraction allocator to address the intrinsic joint stiffness caused by the co-contraction. The proposed controller was implemented on the ankle joints of five healthy subjects to simulate a standing balance situation. The results verified that the proposed controller can achieve desired impedance accurately by adjusting the intrinsic stiffness that stems from the change in the amount of co-contraction (up to 48.4% better impedance achievement with high desired stiffness).

Quadriceps mechanomyography reflects muscle fatigue during electrical stimulus-sustained standing in adults with spinal cord injury - a proof of concept

This study investigates whether mechanomyography (MMG) produced from contracting muscles as a measure of their performance could be a proxy of muscle fatigue during a sustained functional electrical stimulation (FES)-supported standing-to-failure task. Bilateral FES-evoked contractions of quadriceps and glutei muscles, of four adults with motor-complete spinal cord injury (SCI), were used to maintain upright stance using two different FES frequencies: high frequency (HF - 35 Hz) and low frequency (LF - 20 Hz). The time at 30° knee angle reduction was taken as the point of critical "fatigue failure", while the generated MMG characteristics were used to track the pattern of force development during stance. Quadriceps fatigue, which was primarily responsible for the knee buckle, was characterized using MMG-root mean square (RMS) amplitude. A double exponential decay model fitted the MMG fatigue data with good accuracy [R2 = 0.85-0.99; root mean square error (RMSE) = 2.12-8.10] implying changes in the mechanical activity performance of the muscle's motor units. Although the standing duration was generally longer for the LF strategy (31-246 s), except in one participant, when compared to the HF strategy, such differences were not significant (p > 0.05) but suggested a faster muscle fatigue onset during HF stimulation. As MMG could discriminate between different stimulation frequencies, we speculate that this signal can quantify muscle fatigue characteristics during prolonged FES applications.

SVR modelling of mechanomyographic signals predicts neuromuscular stimulation-evoked knee torque in paralyzed quadriceps muscles undergoing knee extension exercise

BACKGROUND AND OBJECTIVE

Using traditional regression modelling, we have previously demonstrated a positive and strong relationship between paralyzed knee extensors' mechanomyographic (MMG) signals and neuromuscular electrical stimulation (NMES)-assisted knee torque in persons with spinal cord injuries. In the present study, a method of estimating NMES-evoked knee torque from the knee extensors' MMG signals using support vector regression (SVR) modelling is introduced and performed in eight persons with chronic and motor complete spinal lesions.

METHODS

The model was developed to estimate knee torque from experimentally derived MMG signals and other parameters related to torque production, including the knee angle and stimulation intensity, during NMES-assisted knee extension.

RESULTS

When the relationship between the actual and predicted torques was quantified using the coefficient of determination (R²), with a Gaussian support vector kernel, the R² value indicated an estimation accuracy of 95% for the training subset and 94% for the testing subset while the polynomial support vector kernel indicated an accuracy of 92% for the training subset and 91% for the testing subset. For the Gaussian kernel, the root mean square error of the model was 6.28 for the training set and 8.19 for testing set, while the polynomial kernels for the training and testing sets were 7.99 and 9.82, respectively.

CONCLUSIONS

These results showed good predictive accuracy for SVR modelling, which can be generalized, and suggested that the MMG signals from paralyzed knee extensors are a suitable proxy for the NMES-assisted torque produced during repeated bouts of isometric knee extension tasks. This finding has potential implications for using MMG signals as torque sensors in NMES closed-loop systems and provides valuable information for implementing this method in research and clinical settings.